Docker: update dependencies, introduce Composer

Docker: update dependencies, introduce Composer

Relates to https://github.com/shaarli/Shaarli/issues/607
Relates to https://github.com/shaarli/Shaarli/pull/612
Relates to https://github.com/shaarli/Shaarli/pull/624
Relates to https://github.com/shaarli/Shaarli/pull/633

See https://github.com/shaarli/Shaarli/wiki/Server-requirements

Modifications:
- [prod][stable] refactor Dockerfile
- [prod][stable] set $TERM=dumb to avoid debconf-related issues
- [prod][stable] install ca-certificates
- [prod][stable] cleanup APT cache after installing packages
- [prod][stable] use Composer to resolve PHP dependencies

Signed-off-by: VirtualTam <virtualtam@flibidi.net>